The present invention relates to a phase modulation multiplexing transmission unit for multiplexing and transmitting a phase modulation signal in a spectrum spread (referred to as, SS hereinafter) transmission system.
FIG. 5 is a block diagram showing a construction of a conventional phase modulation multiplexing transmission unit.
The prior art shown in FIG. 5 comprises a signal division unit 1 for dividing a digital input signal into two division signals I1(n) and I2(n) for outputting and spread code generators 2a and 2b for generating and outputting spread codes C1(n) and C2(n) for spectrum spreading. This prior art further comprises multiplication units 3a and 3b for outputting multiplication signals Sa and Sb obtained by multiplying the division signals I1(n) and I2(n) by the spread codes C1(n) and C2(n) output from the spread code generators 2a and 2b, respectively.
Additionally this prior art comprises an adder 5 for outputting a synthetic signal Sc(n) obtained by summing the multiplication signals Sa and Sb output from the multiplication units 3a and 3b, a modulator 6 for two-phase modulating (referred to as, BPSK hereinafter) the synthetic signal Sc(n) output from the adder 5, a transmission amplifier 7 for amplifying and outputting the phase modulation signal output from the modulator 6 and an antenna 8 through which the amplified phase modulation signal output from the transmission amplifier 7 is transmitted.
The operation of this prior art is described hereinafter.
The signal division unit 1 divides a digital input signal into two division signals I1(n) and I2(n), each of which is input to the multiplication units 3a and 3b, respectively. The spread code generators 2a and 2b generate spread codes C1(n) and C2(n) for spectrum spreading, each of which is output to the multiplication units 3a and 3b, respectively. The multiplication unit 3a multiplies the division signal I1(n) input from the signal division unit 1 by the spread code C1(n) output from the spread code generator 2a. The resultant multiplication signal Sa is obtained from the following equation (1). EQU Sa=I1(n)*C1(n) (1)
The multiplication unit 3b multiplies the division signal I2(n) input from the signal division unit 1 by the spread code C2(n) output from the spread code generator 2b. The resultant multiplication signal Sb is obtained from the following equation (2). EQU Sb=I2(n)*C2(n) (2)
The adder 5 sums the multiplication signals Sa and Sb output from the multiplication units 3a and 3b, respectively for synthesizing. The resultant synthetic signal Sc(n) is obtained from the following equation (3). EQU Sc(n)=Sa+Sb=I1(n)*C1(n)+I2(n)*C2(n) (3)
The modulator 6 two-phase modulates (BPSK) the synthetic signal Sc(n), which is amplified by the transmission amplifier 7 and then transmitted through the antenna 8. The spread codes C1(n) and C2(n) respectively generated by the spread code generators 2a and 2b have excellent self-correlation characteristics using code exhibiting good mutual correlation characteristics (close to non-correlation).
FIG. 6 shows coordinates of the synthetic signal Sc(n) output from the adder 5.
As FIG. 6 shows, the multiplication signals Sa and Sb respectively output from the multiplication units 3a and 3b overlap with each other at coordinates (1, 0) and (-1, 0) on the phase plane. The synthetic signal Sc(n) is defined by signal coordinates (2, 0), (-2, 0) and (0, 0) as shown in FIG. 6. Accordingly the peak level of dualized synthetic signal Sc(n) is doubled, thus increasing the electric power by 4 times (2.sup.2).
FIG. 7 is a graphical representation of an input/output characteristic of the transmission amplifier 7.
Referring to FIG. 7, assuming that the number of multiplexing is 2 (dual), the peak level of the BPSK input/output signal input to the transmission amplifier 7 becomes two times higher than that before multiplexing owing to a high ratio of the average power to the peak power (peak factor). In order to amplify the modulation signal output from the modulator 6 through the transmission amplifier 7 and to transmit the resultant amplified phase modulation signal at a low bias, a broad linear area is required.
Prior arts disclosed by a publication of JP-A-360434/1992 titled "Spectrum spread transmission unit and spectrum spread reception unit" and a publication of JP-A-30079/1993 titled "Spectrum spread modulation unit" have been well known as arts related to the above-described device.
In the publication of JP-A-360434/1992, each bit of parallel data is spread based on a plurality of spread codes and parallel transmitted for spectrum spread transmission at a high rate.
In the publication of JP-A-30079/1993, parallel data converted from serial data are delayed for shifting codes through spread modulation with n delay PN codes which have been phase corrected. As a result, efficient high rate data transmission is realized by preventing degradation in the spectrum spread communication characteristic.
Those conventional phase modulation multiplexing transmission units allow for high rate data transmission. However they need substantially a broad area where the transmission amplifier 7 amplifies the modulation signal output from the modulator 6 at a low bias for transmission. As a result, a large-sized transmission amplifier is necessary, resulting in increasing the cost.